Raman and Infrared Structural Investigation of <i>x</i>Rb₂O·(1 − <i>x)</i>GeO₂ Glasses

Abstract

Infrared reflectance and Raman spectra of rubidium germanate glasses, xRb2O·(1 − x)GeO2, have been measured and studied as a function of Rb2O mole fraction in the range 0 ≤x ≤ 0.60. The spectra of corresponding crystalline germanate phases, GeO2-quartz, GeO2-rutile, Rb2Ge4O9, K4Ge9O20, K2Ge2O5, and K2GeO3, have been also measured for a better understanding of the glass spectra. At very low content (x ≤ 0.01) Rb2O was found to induce a redistribution of GeO4 tetrahedra in rings smaller than those encountered in quartz- and glassy GeO2. The spectral manifestations of this trend are the frequency increase of the Raman active symmetric stretching vibration of Ge−O−Ge bridges and the parallel decrease of the infrared frequency of the asymmetric stretching of Ge−O−Ge bridges. Beyond the composition x = 0.01 and up to x ≈ 0.15−0.20, the predominant modification mechanism involves the transformation of GeO4 tetrahedra into GeO6 octahedra, with characteristic Raman scattering at 315, 590, and 630 cm-1. At higher alkali content and up to the limits of glass formation, tetrahedral Qn species with nonbridging oxygens are formed ((4 − n) is the number of nonbridging oxygens per tetrahedron). Thus, Q3 appears beyond x = 0.10, giving place to Q2 (x > 0.30), Q1 (x > 0.50), and Q0 (x = 0.60). The characteristic Raman bands of these germanate tetrahedra were measured at 865, 765, 740, and 720 cm-1, respectively. Based on semiempirical models, the spectroscopic results were quantified to yield the average coordination number of Ge atoms, as well as the fraction of Ge atoms in 6-fold coordination. Both were found to exhibit a maximum value at approximately 15 mol % Rb2O.